1. Field of the Application
The present invention relates to an oven controlled crystal oscillator (OCXO) capable of obtaining oscillation frequency with a high stability, and more particularly, to a temperature control circuit of an oven controlled crystal oscillator capable of adjusting a temperature of the oven to a peak temperature of the crystal and canceling the temperature gradient.
2. Description of Related Art
[Prior Art]
The operation temperature of the crystal resonator of an oven controlled crystal oscillator is maintained at a constant temperature, thus the frequency, which depends on the frequency-temperature characteristic, maintains unchanged. An oscillation frequency with a high stability is thereby obtained in an OCXO. A crystal resonator is accommodated in a thermostatic oven, and the temperature of the thermostatic oven is controlled at a constant temperature by using a temperature control circuit.
[Temperature Control Circuit of a Conventional Oven Controlled Crystal Oscillator: FIG. 4]
FIG. 4 schematically illustrates a temperature control circuit diagram of a conventional oven controlled crystal oscillator of related art. A temperature control circuit of a conventional oven controlled crystal oscillator is described with reference to FIG. 4. FIG. 4 schematically illustrates a circuit diagram of a temperature control circuit of a conventional oven controlled crystal oscillator. Basically, as shown FIG. 4, the temperature control circuit of a conventional OCXO includes a thermistor TH1, a differential amplifier (OPAMP) IC, a power transistor Q and a heater resistor H1.
[Connection Relationship]
A supply voltage VCC is applied to an end of the heater resistor H1, and the other end of the heater resistor H1 is connected to a collector of the power transistor Q, and an emitter of the power transistor Q is connected to the ground GND.
In addition, supply voltage VCC is also applied to an end of the thermistor TH1, and the other end of the thermistor TH1 is connected to an end of the resistor R1, and the other end of the resistor R1 is connected to an end of the resistor RR, and the other end of the resistor RR is connected to the ground GND.
Moreover, the supply voltage VCC is also applied to an end of the resistor R2, and the other end of the resistor R2 is connected to an end of the resistor R3, and the other end of the resistor R3 is connected to the ground GND. In addition, the supply voltage VCC is applied to the differential amplifier IC and the differential amplifier IC is connected to the ground GND.
Then, a point between the other end of the thermistor TH1 and an end of the resistor R1 is connected to a first input terminal (negative terminal) of the differential amplifier IC via a resistor R4, and a point between the other end of the resistor R2 and an end of the resistor R3 is connected to a second input terminal (positive terminal) of the differential amplifier IC. In addition, the first input terminal of the differential amplifier IC is connected to an output terminal of the differential amplifier IC via a resistor R5. Then, the output terminal of the differential amplifier IC is connected to a base of the power transistor Q via a resistor R6.
[Each Part]
The thermistor TH1 is a temperature sensor whose resistance value varies with temperature and detects the operational temperature of the crystal resonator. In the differential amplifier IC, the voltage between the thermistor TH1 and the resistor R1 is input to the first input terminal (negative terminal) via the resistor R4 with the output terminal of the differential amplifier IC inputting a feedback via the resistor R5, and the voltage between the resistor R2 and the resistor R3 is input to the second input terminal (positive terminal), and the voltage difference between the two input terminals (negative terminal and positive terminal) is amplified.
In the power transistor Q, the output of the differential amplifier IC is input to the base via the resistor R6, and a current flows between the collector and the emitter corresponding to the applied voltage of the base so that a current also flows through the heater resistor H1. The heater resistor H1 generates a heat corresponding to the current flowed. Herein the power transistor Q and the heater resistor H1 become heat sources.
The above mentioned configuration is to maintain the temperature to be constant within the oven. Nevertheless, in order to change the temperature within the oven, the resistance value of the resistor RR is correspondingly changed.
[Frequency-Temperature Characteristic]
The frequency-temperature characteristic of the crystal resonator is a cubic curve. The oven controlled crystal oscillators realize the high stability by adjusting the temperature of the thermostatic oven at a most stable peak temperature (generally 80 to 95° C.). Since the peak temperature is within about 15° C. range, an adjustment of the temperature of the oven by using the resistor RR is required.
Since oven controlled crystal oscillators are to be used in measuring instruments or base stations with high accuracy, or the like for a long time such as 10 years or 20 years, fixed resistors RR are assembled one by one in OCXO. If analog mechanical variable resistors are used in OCXO, the resistance value may vary due to vibration, heat and deterioration of contact surface caused by oxidation. Accordingly, the preset temperature of the thermostatic oven and the frequency may vary, which has become a major problem; thus, the analog mechanical variable resistors are not generally used.
Furthermore, it is easy to determine the peak temperature of the crystal resonator unit during manufacturing. However, once the crystal resonator unit is assembled in an actual oscillation circuit, the peak temperature may generally shift due to the variation of the assembly between the oscillation circuit and the thermostatic oven. The peak temperature cannot be adjusted easily, it requires one-by-one measuring from outside by using a switch and a resistance value change.
[Related Art]
Additionally, Japanese Patent Laid-Open no. 2011-004382 “Temperature-Controlled Crystal Oscillator” (NDK Co., Ltd.) [Patent document 1], Japanese Patent Laid-Open no. 1995-240628 “Control Circuit of Thermostatic Oven and Crystal Oscillator Using the Same” (NDK Co., Ltd.) [Patent document 2], and Japanese Patent Laid-Open no. 2000-183649 “High Stable Piezoelectric Oscillator” (Toyo Communication Equipment Co., Ltd.) [Patent document 3] are the related arts.
In the Patent document 1, the reference voltage input to the input terminal (positive terminal) of the operational amplifier (differential amplifier) 14 is divided into the voltage of linear resistor 12 and the voltage of resistor 13B, wherein the resistance value of the linear resistor 12 is varied corresponding the ambient temperature.
In the Patent document 2, in the control circuit of the thermostatic oven, the voltage input to the input terminal (negative terminal) of the differential amplifier circuit 7 is divided into the voltage of thermistor 10 and the voltage of digital potentiometer 18, wherein the resistance value of the digital potentiometer 18 is set with respect to an external signal.
In the Patent document 3, in the temperature control unit of the high stable crystal oscillator, the voltage input to the gate of the transistor Tr2, which operates the heaters H1 and H2, is divided into the voltage of thermistor Th, the voltage of the transistor Tr3 and digital variable resistor ICRv1, wherein the resistance value of the digital variable resistor ICRv1 can be set externally.
Patent document 1: Japanese Patent Laid-Open no. 2011-004382
Patent document 2: Japanese Patent Laid-Open no. 1995-240628
Patent document 1: Japanese Patent Laid-Open no. 2000-183649
However, in conventional oven controlled crystal oscillators, regarding the shifting of the peak temperature of the crystal resonator after the circuit is assembled, an external adjustment of the resistance value is required, in which the preparation and the measurement are time-consuming during the manufacturing process.
Besides, when a chip resistor is used to adjust a temperature range of 15° C., the resistor is actually assembled in 24 lines or 96 lines and is preferred to have a resistance value matching the peak temperature of the original crystal resonator. However, it is not necessarily to choose the resistance for the peak temperature.
Moreover, when the resistance for adjusting the oven temperature to the peak temperature is replaced by a potentiometer, the potentiometer has a temperature gradient of +100 to 800 ppm/° C.; accordingly, high stability cannot be achieved.
In addition, though the conventional temperature control circuit (shown in FIG. 4) operates to attempt to maintain the temperature inside the oven to be constant, there is still a small temperature variation. Therefore, in order to mitigate the variation, the resistor RR may be replaced by a diode. Diodes have a temperature dependence on the forward voltage and thus the variation can be compensated.
However, the forward voltage of the diode is 0.7 V, and the adjustment voltage is fixed to 0.7 V by using one diode, and is fixed to 1.4 V by using two diodes. The adjustment voltage by using the diode(s) is not appropriate for an adjustment voltage of a low voltage type control circuit, 3.3 V or 2.5 V, which is the mainstream in recent years. Thus, it is difficult to use diode(s) to compensate the temperature variation.
In order to resolve the above-mentioned problem, the Patent document 1 discloses a compensation method which can be used in low voltage. However, as the aforementioned, since the oven controlled crystal oscillators have high stability such as a frequency deviation of 10−9 ppb, furthermore with complicated thermostatic oven structures, each of the structures has a specific temperature characteristic. Thus, even if the calibration method of the Patent document 1 is used, mechanical and individual adjustment is still required, and is time-consuming.
In addition, the modification method of the Patent document 1 can only modify the variation in one direction. The direction that should be modified relates to a plurality of elements; Thus, it cannot be determined by theoretical design but can be determined by trial-and-error, and this trial-and-error method required for adjustment is rather time-consuming.
Additionally, in the Patent document 2, the voltage input into the input terminal of the differential amplifier is divided into voltage of thermistor and voltage of digital potentiometer, and the resistance value of the digital potentiometer can be changed externally. Further, the oscillation frequency can be easily set. However, the temperature gradient of the potentiometer cannot be cancelled.
Furthermore, in the Patent document 3, the voltage input to the gate of the transistor which operates the heaters is divided into voltage of thermistor, voltage of transistor and digital variable resistor, and the temperature of the thermostatic oven can be reduced, and deterioration of the electronic device can also be reduced. However, the temperature gradient of the digital variable resistor cannot be cancelled.
Moreover, in the Patent document 1, for inputting the reference voltage input to the input terminal of the operational amplifier, a linear resistor of which the resistance value varies according to the ambient temperature is disposed, and for the temperature variation, stabilizing the reference voltage is shown in the figures. However, being able to cancel the temperature gradient of the potentiometer is not disclosed.